How do submarines and whales sink and float in water as needed?

Submarines and whales have ways to adjust their buoyancy. That's how they sink and float as needed. Each has a different method of adjusting buoyancy.

The submarine controls buoyancy by adjusting ballast. Submarines have ballast tanks that can be filled with air or flooded down with water. When full of air, the ballast tanks provide enough buoyancy to keep the submarine afloat (provided the pressure hull is not full of water). The ballast tanks are open on the bottom, and have valves (called main vents) on the top. When we open the main vents, air is forced out by sea water that is coming in at the bottom. Once the ballast tanks are flooded, the boat will submerge. The crew then "fine tunes" the buoyancy by adding water to or pumping water out of tanks inside the pressure hull. These tanks are also used to trim the ship so it is neither heavy in the bow or the stern.

Whales can fill their lungs with air to float or swim on or near the surface. When they dive, they take a breath and swim down against the upward force of buoyancy. As they move deeper, pressure forces their ribs (which are not rigidly attached to their vertebrae) inward and their lungs are compressed. This reduces buoyancy and makes it easier to go deeper. As far as actual "control" of their buoyancy, investigators are still not clear about many aspects of the workings of whale physiology, and debate and speculation continue.